1. Field of the Invention
The present invention relates to a III-V group nitride system semiconductor substrate, a method of making the same, a III-V group nitride system semiconductor device, and a lot of III-V group nitride system semiconductor substrate.
2. Description of the Related Art
Nitride system semiconductor materials such as gallium nitride (GaN), indium gallium nitride (InGaN) and gallium aluminum nitride (GaAlN) have a sufficiently wide bandgap and are of direct transition type in inter-band transition. Therefore, they are a great deal researched to be used for short-wavelength light emitting device. Further, they have a high saturation drift velocity of electron and can use two-dimensional carrier gases in hetero junction. Therefore, they are also expected to be used for electronic device.
With silicon (Si) or gallium arsenide (GaAs) which is already in popular use, an epitaxial growth layer of silicon (Si) or gallium arsenide (GaAs) to compose a device is homo-epitaxially grown on Si substrate or GaAs substrate of same kind of material. In the homo-epitaxial growth on homo-substrate, the crystal growth proceeds in step flow mode on the initial stage. Therefore, it is easy to obtain a flat and epitaxially grown surface while generating little crystal defect.
On the other hand, it is difficult to grow a bulk crystal of nitride system semiconductor, and a GaN self-standing substrate for practical use is just developed. At present, a widely used substrate for epitaxial growth GaN is sapphire. The process of growing a nitride system semiconductor epitaxial layer to compose a device is generally conducted as follows. At first, a GaN layer is hetero-epitaxially grown on single-crystal sapphire by using vapor-phase growth such as MOVPE (metal organic vapor phase epitaxy), MBE (molecular beam epitaxy) and HVPE (hydride vapor phase epitaxy). Then, the nitride system semiconductor epitaxial layer is grown on the GaN layer sequentially or in another growth vessel.
Since the sapphire substrate has a lattice constant different from that of GaN, single-crystal film of GaN cannot be obtained by growing GaN directly at a high temperature on the sapphire substrate. Thus, a method is invented that AlN or GaN buffer layer is in advance grown on the sapphire substrate at a low temperature of 500° C. or so, thereby reducing the lattice strain, and then GaN is grown on the buffer layer (e.g., Japanese patent application laid-open No.4-297023). With such a low temperature growth buffer layer, it becomes possible to obtain single-crystal epitaxially grown GaN. However, even in this method, the lattice mismatch between the sapphire substrate and the grown crystal is not eliminated and, at the initial step of growth, the crystal growth proceeds in three-dimensional island growth mode (Volmer-Waber growth mode), not in step flow mode (Stranski-krastanov growth mode) aforementioned. Therefore, GaN thus obtained has a dislocation density as much as 109 to 1010 cm−2. Such a defect causes a problem in fabricating GaN system device, especially LD or ultraviolet emission LED.
In recent years, ELO (e.g., Appl. Phys. Lett. 71 (18) 2638 (1997)), FIELO (e.g., Jpan. J. Appl. Phys. 38, L184 (1999)) and pendeoepitaxy (e.g., MRS Internet J. Nitride Semicond. Res. 4S1, G3.38 (1999)) are reported that are methods for reducing a defect density generated due to the lattice mismatch between sapphire and GaN. In these methods, a SiO2 patterning mask etc. is formed on GaN grown on a sapphire substrate, and then GaN is selectively grown from the mask window. Thereby, the propagation of dislocation from underlying crystal can be suppressed. Due to such a growth method, the dislocation density in GaN can be significantly reduced to a level of 107 cm−2 or so. For example, Japanese patent application lain-open No.10-312971 discloses such a method.
Further, various methods of making a self-standing GaN substrate are suggested that a thick GaN layer with reduced dislocation density is epitaxially grown on a hetero-substrate such as sapphire and then the GaN layer grown is separated from the underlying substrate (e.g., Japanese patent application laid-open No.2000-22212). For example, Japanese patent application laid-open No.11-251253 discloses a method of making a self-standing GaN substrate that a GaN layer is grown on a sapphire substrate by ELO and then the sapphire substrate is removed by etching. Other than this, VAS (Void-Assisted Separation: e.g., Y. Oshima et al., Jpn. J. Appl. Phys. Vol.42 (2003) pp. L1-L3, Japanese patent application laid-open No.2003-178984) and DEEP (Dislocation Elimination by the Epi-growth with inverted-Pyramidal pits: e.g., K. Motoki et al., Jpn. J. Appl. Phys. Vol. 40 (2001) pp. L140-L143, Japanese patent application laid-open No.2003-165799) are known. The VAS is conducted such that GaN is grown through TiN thin film with a mesh structure on substrate such as sapphire while providing voids at the interface of underlying substrate and GaN layer, thereby allowing both the separation and the dislocation reduction of GaN substrate. The DEEP is conducted such that GaN is grown on a GaAs substrate, which is removable by etching, by using a SiN patterning mask while intentionally forming pits surrounded by facets on the surface of crystal, accumulating dislocations at the bottom of pits to allow regions other than pits to have a low dislocation density.
However, the conventional methods of making GaN substrate have next problems.
As described above, a GaN crystal to compose a GaN self-standing substrate is at least once hetero-epitaxially grown on the hetero-substrate such as sapphire and GaAs with a considerably different lattice constant. The GaN crystal grown on the hetero-substrate is subjected to a bowing caused by a difference in lattice constant or linear expansion coefficient between the GaN crystal and the underlying hetero-substrate. It is known that such a bowing is significantly observed even in a self-standing substrate after removing the underlying substrate. In some cases, such a bowing may be generated already during the crystal growth, where the crystal continues growing while being kept bowed. In the other cases, a crystal may grow while retaining such a strain inside thereof and the bowing may be generated after removing the underlying substrate. For example, Japanese patent application laid-open No.2000-22212 discloses an example that a convex-upward bowing is generated in a GaN self-standing substrate manufactured using a GaAs substrate as underlying substrate (FIGS. 11 and 15 ibid.).
When the GaN substrate is bowed, the crystal axis thereof also has an in-plane distribution according to the bowing. This is also indicated in FIG. 15 of Japanese patent application laid-open No.2000-22212.
GaN self-standing substrates are frequently marketed in the form of having its surface mirror-finished by polishing as in other semiconductor substrates. Therefore, although they may appear to be flat, a distribution in inclination of crystal axis may be generated due to the bowing of the original GaN substrate before the polishing.
This situation will be explained below with reference to drawings.
FIG. 1 is an illustrative cross sectional view showing the definition of parameters to represent an inclination direction of crystal axis. Provided that, at an arbitrary point A, a low index surface 15 closest to a substrate surface 14 has an inclination to the substrate surface 14, the inclination of crystal axis can be known by finding what direction and how much the normal vector 16 of the low index surface 15 closest to the substrate surface 14 is inclined from the normal line of the substrate surface 14. This can be easily known by X-ray diffraction measurement. What direction the original substrate before the polishing is bowed can be known by finding what direction in the plane of substrate a vector 17 heads in that is made by projecting on the substrate surface 14 the normal vector 16 of the low index surface 15 closest to the substrate surface 14.
FIG. 2 is an illustrative cross sectional view showing a distribution in inclination of crystal axis inside a conventional GaN substrate with a surface that, though it was originally convex-bowed, is flattened by polishing. FIG. 3 is an illustrative top view showing an in-plane distribution of a vector made by projecting on the substrate surface the normal vector of a low index surface closest to the substrate surface in order to show a distribution in inclination of crystal axis viewed from the substrate surface with respect to the conventional GaN substrate with a surface that, though it was originally convex-bowed, is flattened by polishing.
When the original substrate before the polishing is convex upward-bowed to the surface, the crystal axis has a distribution spread on the surface side inside the substrate as shown in FIG. 2 even in a substrate 18 with the surface flattened by polishing. Lines 19 depicted inside the substrate 18 indicate the direction of crystal axis (normal line of low index surface closest to the substrate surface). The substrate 18 has such a distribution that a vector made by projecting on the substrate surface the abovementioned normal vector of low index surface closest to the substrate surface is, as shown by arrows 20 in FIG. 3, radiated to the outside of the substrate 18.
If an epitaxial layer of AlGaN mixed crystal is grown on a GaN substrate with such a distribution in inclination of crystal axis, there occurs a large dispersion in morphology or crack generation of the AlGaN mixed crystal. Thus, the reliability of epitaxial layer grown on the GaN substrate is low. The same tendency is found even when using a GaN layer that is once homo-epitaxially grown on the GaN substrate. This problem is not found in the other semiconductor materials such as Si and GaAs. In other words, it is a unique problem on III-V group nitride system semiconductor layers that are grown using a thick-film substrate that is hetero-epitaxially grown on the hetero-substrate.